add teraslice_exporter_info metric and improve teraslice_exporter_errors

[godber]

We should add an info metric, mostly so we can see the version, and instance info. Looking at the other metrics, it seems that url and name are the labels used for the instance info

teraslice_exporter_info{version="v0.4.0", url="http://ts-WHATEVER.example.com/",name="teraslice-WHATEVER"} 1

furthermore, we should add the url and name to the recently added teraslice_exporter_errors:

teraslice_exporter_errors{error_type="update_metrics_errors", url="http://ts-WHATEVER.example.com/",name="teraslice-WHATEVER"} 0